The ride quality and operator comfort of a work vehicle is adversely affected by vibrations or movement transmitted from the frame or chassis of the vehicle to the operator's cab. As the work vehicle travels across a surface, movement of the chassis induces the operator's cab to pitch, roll and bounce. Movement of the cab can be particularly severe in agricultural and construction equipment vehicles (e.g., tractors, combines, backhoes, cranes, dozers, trenchers, skid-steer loaders, etc.) because such vehicles typically operate on off-road surfaces or fields having a high level of bumpiness.
Operator comfort may also be adversely affected by the operation of various systems on a work vehicle. In particular, operation of various work vehicle systems can cause forces to be applied to the chassis of the vehicle which, in turn, are transmitted to the cab. Examples of these forces include the following: draft forces exerted on the hitch of an agricultural tractor by an implement (e.g., a plow) which can cause the cab to pitch; normal forces applied to a work vehicle as the vehicle turns in response to a steering device which can cause the cab to roll; clutch forces generated when a work vehicle clutch (e.g., a main drive clutch; four-wheel drive clutch) is engaged or disengaged which can cause the cab to pitch; gear shift forces generated when a transmission of a work vehicle is shifted which can cause the cab to pitch; braking forces generated as brakes of a work vehicle are operated which can cause the cab to pitch; acceleration forces generated when a speed actuator changes the speed of a work vehicle which can cause the cab to pitch; etc.
The movement of the cab caused by surface bumps and the operation of vehicle systems cause both qualitative and quantitative problems. An operator of such a vehicle experiences increased levels of discomfort and fatigue caused by the vibrations. Productivity is decreased when an operator is forced to rest or shorten the work day, or is unable to efficiently control the work vehicle. The operator is also less likely to be satisfied with a work vehicle which provides poor ride quality. Under certain conditions, the frequency and magnitude of cab movement may force the operator to decrease driving speed, further decreasing productivity.
To improve ride quality and operator comfort, work vehicles have been equipped with passive, semi-active or active suspension systems to isolate the operator from vibrations caused by surface bumps. Such systems include vibration isolators mounted between the chassis and cab or seat. Passive systems use passive vibration isolators (e.g., rubber isolators, springs with friction or viscous dampers) to damp vibrations with different isolators used to damp different frequencies. Rubber isolators may be used, for example, to damp high frequency vibrations and air bags used to damp low frequency vibrations. However, performance of passive systems is limited due to design compromises needed to achieve good control at resonance frequencies and good isolation at high frequencies.
Semi-active systems achieve control and isolation between the chassis and the cab by controlling a damper to selectively remove energy from the system in response to movement of the cab sensed by sensors. Active systems use sensors to sense cab movement and a controller to generate control signals for an actuator which applies a force to the cab to cancel vibrations transmitted to the cab by the chassis. The power needed to apply the force is supplied by an external source (e.g., hydraulic pump).
As the above paragraphs imply, it is desirable that a suspension system attenuate both low and high frequency vibrations between the chassis and cab. Attenuation of high frequency vibrations can decrease acoustic noise in the cab, decrease fatigue and decrease vibration-induced mechanical faults. Attenuation of low frequency (e.g., less than 20 Hz) vibrations can decrease operator fatigue and improve vehicle operability. The attenuation of low frequency vibrations is particularly important because the resonant frequencies of the human body are typically below 20 Hz. For example, the human abdomen resonates at frequencies between 4-8 Hz, the head and eyes resonate at frequencies around 10 Hz, and the torso at 1-2 Hz. The actual frequency may vary with the particular individual.
Existing active suspension systems use a fixed or manually-adjustable gain. The gain determines the level of force applied to the cab by the actuator to cancel the vibrations transmitted to the cab by the chassis. Vibration isolation is maximized by setting a maximum gain. However, the mechanical limits of the active actuators (e.g., fixed stroke length of hydraulic cylinders) impose a limit on the gain. Exceeding the limit could cause the system to saturate by causing the cylinder's piston to move beyond the fixed stroke length and hit the cylinder's mechanical stops. Thus, it would be desirable to have an active suspension system wherein the gain is automatically adjusted to the maximum value which will not cause the active actuator to exceed its stroke length.
One active suspension system for a work vehicle includes a hydraulic actuator mounted at a single point between the rear of the cab and the vehicle frame. The front of the cab is pivotally mounted to the frame. The actuator is controlled to move the cab relative to the frame in response to sensed acceleration signals. The system includes a single air bag used to level the cab. This system, however, only affects cab pitch since the actuator can only pivot the cab about the single point.
Another active suspension system for a work vehicle includes one active vibration isolator mounted between the vehicle chassis and the rear of the cab, and two active isolators mounted between the chassis and the front of the cab. Each isolator includes a hydraulic actuator mounted between the chassis and the cab, and an air bag to support the weight of the cab. The actuator is controlled to move the cab relative to the chassis in response to sensed acceleration signals. Each isolator is individually controlled by an electronic controller replicated for each isolator. The transfer function of the controller is pre-tuned to the dominant frequency of the chassis, and includes a manually adjustable gain. However, the performance parameters of the isolators are not automatically adjusted.